lib/nfs/Connection: remove deprecated move constructor workaround

Not used anymore because NfsManager now uses boost::intrusive::set
instead of std::map.
lib/nfs/Connection: remove obsolete flag postponed_destroy
lib/nfs/Connection: add method GetEventLoop()
lib/nfs/Connection: remove Mutex

All locks are currenly held from only a single thread (the IOThread)
and thus we don't need the Mutex.
lib/nfs/Connection: fix memory leak when cancelling Open()

Close the newly allocated file handle passed to the callback.
lib/nfs/FileReader: postpone the nfs_close_async() call

If an async opertion is in progress, nfs_close_async() will make
libnfs crash because the RPC callback will dereference an object that
was freed by nfs_close_async().